Not adept enough to do this myself or even evaluate how viable it may be, but I can’t help but feel like the Marathon games by Bungie (best known for Halo) would be a great add to the ports section of RetroPie.
All of the games have been released for free and run on the open source Aleph One engine, which is already available on Linux. Not sure how much energy optimizing that engine for the Pi would be, but I love these games, and I’m sure I’m not the only one.